Following situation came up in a local rec league game I was watching. Hard fought match where, IMO, perhaps a few more whistles earlier might have kept it under control. Late in the match, I think blue up by a goal, corner for red. Ball played into the area, red forward goes down (IMO he appeared to dive, but it was ruled a foul). Penalty for red signalled. Blue defender is upset at the dive, and pushes the red forward. Red forward retaliates with a punch before the referee can break up the fight. Both the push and the punch occured in the area. Both players are going to get sent off under any interpretation, I'd assume. But what is the proper restart? Does this change if the push and punch took place outside the area? Does this change if the red forward was smart enough not to retaliate?
The restart is always determined by what reason the game was stopped, regardless of what happens in between. So in your case it still would be a PK for the red team.
The ball is out of play with a restart of a PK. The restart cannot change [without new information from an AR that the referee accepts]. This is yahaddabethere. In this case no cards are "mandatory." However, "extremely likely" is a good phrase. The blue player has probably earned a caution for USB or a sendoff for VC. It depends on what the push looks like, how hard, etc. Your description sounds like USB to me. The red player has probably earned a sendoff for VC or a caution for USB. It depends on what the punch looks like, where aimed, how hard, etc. Your description sounds like VC to me.